The present invention relates to a multi-layer, unsupported, integral microfiltration (MF) membrane of polymeric material, with the exception of aliphatic polyamide, that is suitable for a phase inversion process, the MF membrane having at least one final filter layer and at least one integrated prefiltration layer. The present invention also relates to a method of making such a membrane.
Microfiltration membranes are known. Such membranes generally have pore diameters in the range of from 0.01 to 10 .mu.m. Microfiltration membranes can be classified as follows:
a) Symmetrical membranes where the diameters of the micropores remain practically constant throughout their thickness, so that both sides have nearly the same pore diameters;
b) Asymmetrical membranes, where a thin 0.1 to 0.25 .mu.m dense skin layer is placed upon and integrally bonded to a porous substrate member. The skin determines not only the permeability but also the permselectivity of the dual layer membrane, while the porous substrate member essentially serves as a support structure for the skin;
c) Composite membranes, which in principle constitute an asymmetrical membrane having a skin, with the skin and the support structure being made of different polymeric materials. The support material is already embodied as an asymmetrical membrane upon which the thin skin layer is then disposed.
Whereas composite membranes are essentially used for reverse osmosis, pervaporation, and gas separation, and less frequently for microfiltration purposes, symmetrical and asymmetrical membranes are frequently utilized in the microfiltration industry.
Although symmetrical microfiltration membranes generally offer a high retention rate and are also relatively reliable due to their homogeneous membrane structure, they nonetheless have a low flow rate since they offer a great resistance to the fluid stream during filtration. Due to the relatively low porosity and to the approximately two dimensional surface configuration, such membranes unfortunately tend to clog up relatively rapidly, which is reflected in a limited sludge carrying capacity (fouling), so that the useful filtration time is very short (DE-OS 37 01 633). In contrast, asymmetrical membranes offer a long service time if the flow is on the coarse-pored side since the smallest particles that are to be filtered out can be essentially caught by the density of the skin and hence the entire thickness of the membrane can be effectively utilized, thus increasing the flow rate. However, since the dense skin is disposed at the surface, scratches can easily occur there, thus significantly reducing the reliability and the retention rates (DE-OS) 37 40 871).
EP 05 94 007 A1 discloses an integral composite membrane for microfiltration where a separating layer is at a later stage poured onto a prefabricated symmetrical membrane that is to serve as a prefiltration layer. The thickness of this separating layer ranges from 2 to a maximum of 40 .mu.m.
Due to the method of manufacturing disclosed in EP 05 94 007, it is clear to one skilled in the art that the subsequently poured-on separating layer must relate to an asymmetrical layer. For example, it is indicated that the solution for effecting the coagulation is contacted by a nonsolvent; otherwise, no nonsolvent fraction in the polymer solution is mentioned in this reference. Pursuant to Staude "Membranen und Membranprozesse", Weinheim 1992, page 10 et seq, it is made clear that both conditions indicate the formation of asymmetrical membranes. However, it is evident that such membranes have relatively poor useful lives.
In DE-OS 37 01 633 and DE-OS 37 40 871 the drawbacks discussed in connection with asymmetrical membranes, the fragile separating layer (skin) is disposed in the interior of the membrane, and hence is better protected, via a controlled step in a dry-wet process. However, as before there is still the drawback of a relatively thin separating layer, so that faults that might be present in the membranes, such as oversized pores, can significantly impair the retention characteristic of the membranes. In EP 0 083 489 B1, the problems related to the sensitivity of the skin of asymmetrical microfiltration membranes are addressed by superimposing two asymmetrical membranes upon one another in such a way that their skins contact one another. However, this approach has the drawback that the manufacturing process is relatively expensive and complicated, and furthermore the pore characteristic established by the pore sizes of the skin relative to the exclusion magnitude of particles varies as a consequence of the static arrangement of respectively oppositely disposed pores or matrix material.
If fine-pored microfiltration membranes having nominal pore sizes of less than 0.45 .mu.m are installed in filter cartridges, the porosity of these membranes at their edges is greater than the porosity at their surfaces. As a consequence, the filtration characteristics established by the pore characteristic of the membrane are not achieved. Thus, DE-PS 3 818 860 utilizes a combination of at least three membranes having different materials and different pore sizes and pore geometries, so that the coarse-pored membrane performs the function of a preliminary filter and the fine-pored membrane performs the function of a final filter. Thus, with this hybrid technology fine-pored microfiltration membranes having nominal pore sizes of less than 0.45 .mu.m can also be used in filter cartridges. However, the drawback is that during installation in the filter cartridges, the membranes that are placed one upon the other are not as easy to handle as are individual membranes. When the filter cartridges are sterilized with steam, the outer membrane layers can swell, which leads to damage. U.S. Pat. No. 4,340,480 discloses an individual multi-layer membrane having integral individual layers where the individual layers are pressed upon one another in a wet state and are dried together. However, this method of manufacture is possible only with such polymers that have a high water absorbtiveness, such as polyamide 66, which is used in this reference and has a water absortiveness (saturation) of 8.5% by weight.
It is therefore an object of the present invention to provide microfiltration membranes which, as individual membranes, are built up from two or more integral layers, which while exhibiting a high retention rate and a high reliability at the same time have a high sludge carrying capacity (service life), which are economically made of the same membrane material, whereby polymeric material having a low water absortivity can also be used, and which, with a nominal pore size of below 0.45 .mu.m, can also be used in membrane filter cartridges without difficulty.